1. Field of the Invention
This invention relates to graphite powders having a novel structure suitable as a carbonaceous material for a negative terminal of a lithium ion secondary battery. More particularly, it relates to graphite powders that are able to fabricate a negative electrode of a lithium ion secondary-battery having a high discharge capacity and superior charging/discharging efficiency, a method for producing these graphite powders, a material for a negative electrode of the lithium ion secondary battery formed of these graphite powders, and a lithium ion secondary battery having the negative electrode which is fabricated from this negative terminal material.
2. Description of Related Art
A lithium secondary battery is among non-aqueous secondary batteries employing lithium as an active material for a negative electrode, an oxide of a transition metal or chalcogenides, such as sulfides or selenides, as an active material for the positive electrode, and a solution of an inorganic or organic lithium salt in a non-protonic organic solvent, as an electrolytic solution.
Since lithium is a metal having an extremely base potential, it is possible with the battery employing this as a negative electrode to take out a large voltage easily. Consequently, a lithium secondary battery is recently stirring up notice as a secondary battery of high electromotive force and a high energy density, such that expectations are made of applications thereof as a distribution or portable type battery in a wide range of applications, such as electronic equipments, electric cars or power storage. It is already being put to use as a small-sized battery.
In an early version of the lithium secondary battery, use is made of a foil-shaped metal lithium as a negative electrode material. In this case, a charging/discharging reaction proceeds by dissolution (ionization) and precipitation of lithium. However, since metal lithium tends to be precipitated as a needle on the negative electrode in the reaction of Li+→Li, repeated charging/discharging leads to precipitation of a dendritic lithium (lithium dendrite) on the surface of the negative electrode. If growth of this lithium dendrite is allowed to proceed, shorting with the negative electrode tends to occur through a separator (partition), thus leading to a fatal defect of an extremely short repetitive charging/discharging cyclic life.
As means for solving the problem of the lithium secondary battery, it is proposed in, for example, Japanese Laying-Open Patent S-57-208079 to use a carbon material capable of storing and yielding lithium ions, such as natural graphite, artificial graphite, petroleum coke, sintered resin, carbon fibers, pyrocarbon, carbon black etc, as a negative electrode material. In this case, the negative electrode material may substantially be formed only of the carbon material, and an electrode operating as a negative electrode usually can be fabricated by allowing powders of the carbon material to be deposited on a metal current collector along with a suitable resin binder.
Although the electrode reaction of a lithium secondary battery, the negative terminal of which is prepared from this carbonaceous material, is not known precisely, it may be presumed that, during charging, electrons are forwarded to the carbon material of the negative electrode and charged to the negative polarity such that lithium ions in the electrolytic solution are accumulated by electrochemical intercalation in the carbon material of the negative electrode charged to the negative polarity. Conversely, during the discharging, lithium ions are desorbed (de-intercalated) from the carbon material of the negative electrode and emitted into the electrolytic solution. That is, charging/discharging occurs due to accumulation and emission of lithium ions in or from the negative electrode material. Therefore, this sort of the battery is generally termed a lithium ion secondary battery. In the lithium ion secondary battery, in which metal lithium is not precipitated during the electrode reaction, there is raised no problem of deterioration of the negative electrode due to dendritic precipitation. The lithium secondary battery now in use is mainly of this type, that is, a lithium ion secondary battery the negative electrode of which is formed of a carbon material.
The theoretical capacity of the lithium ion secondary battery, the negative electrode of which is formed only of metal lithium, is as high as approximately 3800 mAH. Conversely, the theoretical capacity of the lithium ion secondary battery, the negative electrode of which is formed of a lithium/graphite interlayer compound (C6Li), is 372 mAH/g, this capacity being retained to be a limit or threshold capacity. It is noted that the lithium/graphite interlayer compound (C6Li) is an inter-layer compound in which lithium ions are packed densely in a regular pattern between layers of graphite which is the most crystalline carbonaceous material.
However, since surface activated sites which inhibit intrusion of lithium ions into the carbon material of the negative electrode and a dead zone against packing of lithium ions exist in actuality in the carbon material of the negative electrode, it has been extremely difficult to achieve the threshold capacity of 372 mAH/g even with the use of the high crystalline graphite as the carbon material for the negative electrode of the lithium ion secondary battery.
Meanwhile, the carbon material may be classified into hard carbon (low-crystalline amorphous carbon) and soft carbon (high-crystalline graphite carbon). The above-mentioned threshold capacity, which holds for the soft carbon, fails to hold for the hard carbon, there being a material manifesting a higher capacity per weight. However, the capacity per volume is lowered because of the lower density of the hard carbon.
If the graphite, as the high-crystalline carbon material, is used as the negative electrode material, there is deposited an inactivated skin film in the course of charging with the above-mentioned decomposition of the electrolytic solution. Since the electrical quantity used at this time represents the loss, the charging/discharging efficiency [discharging capacity/charging capacity×100 (%)], as one of battery indices, is lowered. This is a considerable demerit for a usage such as a small-sized battery having a pre-set shape standard because the quantity of the negative electrode material needs to be estimated to a larger value at the time of battery designing.
For approaching the discharging capacity of the lithium ion secondary battery to the above-mentioned threshold capacity as much as possible, various proposals have so far been made as to the manufacturing method for the carbonaceous material for the negative electrode.
For example, it is proposed in Japanese Laying-Open Patent H-4-115458, Japanese Laying-Open Patent H-5-234584 and Japanese Laying-Open Patent H-5-307958 to use carbides of mesophase globules generated in the pitch carbonization process. The mesophase globules are spherically-shaped particles exhibiting optical isomerism (properties of liquid crystal) and which are generated on heat treatment of pitches for several hours at approximately 400 to 550° C. On continued heat treatment, the globules grow in size and coalesce to become a bulk mesophase which exhibits optical isomerism in their entirety. This bulk mesophase can also be used as the carbon material. However, the discharging capacity of the lithium ion secondary battery employing this negative electrode material is as yet rather low.
In the Japanese Laying-Open Patent H-7-282812, attempts are made to raise the regularity of the layered disposition of the graphite layers in association with graphized carbon fibers to raise the capacity of the lithium ion secondary battery. In this publication, it is stated that, on pulverizing the carbon fibers, undesirable structural defects different from the regular layer disposition of the graphite layers of the original carbon fibers are introduced, such that, for raising the capacity of the lithium ion secondary battery, it is meritorious to raise the regularity of the layered disposition of the graphite layers. However, if the regularity of the layered disposition of the graphite layers is raised in this manner, the discharging capacity of the lithium ion secondary battery is 3.16 mAH/g at the maximum, such that it is not possible to obtain a negative electrode material of the graphite-based carbonaceous material having the capacity as high as 320 mAH/g or higher.
In Japanese Laying-Open Patent H-6-187972, there is disclosed a carbon material obtained on firing, at an elevated temperature, a resin obtained in turn by reacting aromatic components with a cross-liking agent in the presence of an acid catalyst. This carbon material has a structure in which a crystal area of crystallized aromatic components and an amorphous area of amorphized cross-linking agents co-exist and, due to the differential thermal expansion/contraction coefficients between the two, numerous internal structural defects are manifested. It is stated that not only lithium ions are introduced into an inter-layer area to form C6Li, but also metal lithium is occluded int these structural defects, as a result of which it is possible to constitute high-capacity lithium ion secondary battery. However, since a special resin is used as a starting material, the cost of the material is high, thus producing economic demerits. Moreover, since the carbonaceous material is the hard carbon, the capacity per unit volume is lowered. In addition, with this material, the charging/discharging efficiency cannot be improved.
In the Japanese Laying-Open Patent H-3-245548, there is disclosed a carbonaceous material obtained on carbonizing an organic material. This material uses a costly organic resin material, in particular the phenolic resin, as the carbonaceous material, thus raising the cost for the material.
This carbonaceous material is stated as exhibiting a high discharging capacity per unit weight exceeding the threshold capacity of 372 mAH/g for graphite. However, since this material also is hard carbon, the true density is lower, specifically of the order of 1.55 g/cc. On the other hand, the true density of graphite is as high as approximately 2.2 g/cc. Therefore, the discharging capacity per unit volume of the above-mentioned carbonaceous material is as low as 380 mAh/g×1.55 g/cc=589 mAh/cc, in comparison with the discharging capacity per unit volume of the graphite-based material, even though the latter has a lower discharging capacity of, for example, 320 mAH/g. As a consequence, the hard carbon material suffers from the problem that the battery cannot be reduced in size, such that the graphite-based material is more favorable for reducing the battery size because of its high true density.
The present invention envisages to provide a graphite-based material of high true density which is suited for a negative electrode material of a small-sized high-capacity lithium ion secondary battery, even though a carbon material similar to a conventional carbon material is used in place of special resins for carbonization, and a manufacturing method thereof
The present inventors have proposed a high-performance negative electrode material in which the carbon network layer (graphite c-planar layer) has a looped closed structure on the powder surface and in which the density of the interstitial planar sections between the looped closed structures along the graphite c-direction may be controlled to realize a charging/discharging capacity exceeding 320 mAH/g. However, as will now be explained, this negative electrode material is in need of a high-temperature heat treatment at a temperature exceeding 2500° C. for graphization, as before, while a still higher temperature exceeding 3000° C. is required for realizing a higher capacity, such that further improvement is required for application to industrial mass production.
FIG. 1 shows the relation between the discharging capacity and d002 (FIG. 1a) and that between d002 and the graphization temperature (FIG. 1b) in case the bulk mesophase obtained from the petroleum pitch is pulverized, carbonized and subsequently graphized by changing the temperature. It is noted that d002 is the distance between c-axis planar lattices (interlayer distances).
It is seen that d002 is decreased with rise in the graphization temperature and that, with decrease in d002, the discharging capacity is increased. This relation between the discharging capacity and d002 is reported in, for example, Iizima et al, Synth. Met., 73 (1995), 9, from which it is seen that approaching d002 to close to that of natural graphite to raise the capacity is a commonplace technique in the graphite-based negative electrode material (d002 of ideal natural graphite=3.354 Å).
However, in order to obtain a graphite material with d002=3.360 Å, the graphizing heat treatment at an elevated temperature of the order of 3000° C. is required, as may be seen from FIG. 1b. Thus, the graphite-based negative electrode material with a smaller value of d002, that is with a higher performance, cannot be obtained if only the measures of elevating the temperature of the graphizing heat treatment is resorted to.
Meanwhile, from the disturbed carbon network (condensed poly-cyclic structure of six members of carbon), the microscopic process of graphization may be envisioned as being a process of ordering of the arrangement of carbon atoms to a layered graphite phase.
FIG. 2 shows an example of a disturbed network of carbon clusters obtained by a molecular dynamic method employing the Tersoff potential [J. Tersoff, Phys. Rev. Lett., 19, 2879 (1988)]. The system of FIG. 2 is a network with a potential approximately 1.3 eV higher per atom than the structural energy of graphite. In FIG. 2, an arrow indicates a sp3 (four ligancy) carbon atoms different from sp2 (three ligancy) carbon in the graphite. In the disturbed carbon network, the presence of carbon atoms with different numbers of ligands may be easily estimated from the following considerations.
FIG. 3 shows the relation between the pressure and the Gibbs free energy (enthalpy) at 0 K of diamond and graphite as calculated using the Tersoff potential. It is noted that diamond and graphite represent typical examples of the sp3 (four ligancy) network and sp2 (three ligancy) network, respectively. As may be seen from FIG. 3, the four ligancy carbon network and the three ligancy carbon network are stable at high pressure and at low pressure, respectively, with the two being approximately equal to each other in energy and stabilized at a zero pressure.
A wide variety of carbon materials are produced industrially, and a wide variety of structures of the carbon materials have been found. The reason is that, with the structure of the carbon material, a wide variety of combinations of the two networks of substantially equally stable sp3 (four ligancy) and sp2 (three ligancy) are possible. It may be estimated from FIG. 3 that four ligancy network and the three ligancy network are generated in the portion of a run-of-the-mill carbon material subjected to compressive distortion and to that subjected to the tensile distortion, respectively.
The process of graphization is the process of solid-phase growth from the disturbed carbon network, shown in FIG. 2, to the laminar planar carbon structure (three ligancy network). This process is felt to be accompanied by extinguishment of the four ligancy carbon and ordering to a three ligancy network. For example, for changing from the disturbed carbon network as shown in FIG. 2 to the planar three ligancy network, two elementary processes, namely (1) cutting of the bond of the four ligancy carbon and (2) correcting the bond angle and the bond length to sp2 (three ligancy) system. This may be presumed to be accompanied by a significant activation energy.
The process of graphization is now explained a little more theoretically. An experimental value of d002 in natural graphite is 3.3545 Å, with d002 of synthetic graphite gradually approaching that of natural graphite by raising the graphization temperature (see FIG. 1b). Since graphite represents the most stable state, as does diamond, insofar as the element carbon is concerned, it may be presumed that, in the carbon material, there exists a structural energy function for a status parameter (<d002) as shown in FIG. 4 in the carbon material. If such relation between d002 and the structural energy is presupposed, the behavior of d002 and the graphization temperature as shown in FIG. 1b can be explained qualitatively as follows: That is, the higher the temperature, the higher becomes the possibility of the energy barrier ΔE (see FIG. 4) being surpassed thus enabling transition to crystallinity close to natural graphite.
On the other hand, the existence of hard carbon, representing the negative electrode material for the lithium ion secondary battery hand-in-hand with the graphite-based carbon material, may be presumed as follows: That is, in certain carbon network, the energy barrier ΔE cannot be surpassed at a temperature corresponding to the graphization temperature, thus resulting in a minimum energy value remote from that of the natural graphite. This energy barrier ΔE is the activation energy accompanying the growth of the of the planar three ligancy network for the graphite from the above-mentioned disturbed network, specifically the energy barrier required for bond re-arrangement and re-coordination. Specifically, this model indicates that re-coordination of the carbon network represents the speed-regulating stage of graphization (graphite solid phase growth).
In the elementary process of graphization, it is necessary to cut the linkage of the four ligancy carbon. This may be presumed to be accompanied by a, extremely large activation energy. Thus, the present inventors directed attention to the III group elements that can form three σ bonds. The reason is that, if the amount of the four ligancy element carbon of the disturbed carbon network can be reduced by substitution by three ligancy elements, the activation energy is diminished, so that, from the above considerations, there is a possibility of the graphization temperature being changed significantly by small changes in the activation energy. There is, however, a problem raised as to whether or not, in the graphite network following graphization, the III group element can substitute the carbon element without disturbing the planar structure.
If, in the lithium ion secondary battery, the graphite-based carbon material is used as the negative electrode material in the lithium ion secondary battery, the charging/discharging reaction takes place by intercalation of lithium ions to the negative electrode material. If three ligancy elements are substituted such as to disturb the planar structure, the risk is high that lithium ion intercalation is obstructed. Thus, the present inventors have searched, by the molecular orbit method, into stability of the three ligancy elements in the graphite network, and have ascertained by the computational chemical technique that boron can be substituted for carbon without disturbing the graphite planar section, as shown in FIG. 5.
Thus, the present inventors have surmised that, if boron that can be substituted for carbon without disturbing the graphite planar section is added and graphization heat treatment is carried out, this element would act as a sort of a catalyst to render it possible to produce graphite with small d002 at a lower energy (that is at a lower heat-treatment temperature) than conventionally. This point was confirmed by an experiment.
FIG. 6 shows an example of the relation between the inter-layer distance d002 and the graphization temperature of graphite samples obtained on heat-treatment at various graphization temperatures of an as-carbonized carbonaceous material admixed with boron and the same material not admixed with boron. With the material admixed with boron, a small value of d002 can be realized with a graphization heat treatment at a lower temperature, with the rate of change of d002 with respect to the graphization temperature being lower than that with the material not admixed with boron. That is, it has been found that, with the material admixed with boron, it is possible to produce a negative electrode material with a lower value of d002 and hence of a larger capacity than that produced with the conventional high temperature heat-treated material.
The present inventors have confirmed that if, in the previously proposed graphite-based negative electrode material having a looped closure structure of the carbon network layer on the surface, the carbon material is subjected to graphizing heat treatment after addition of B, a negative electrode material with a higher performance can be produced inexpensively at a lower graphization temperature, and a negative electrode material of a higher performance can be produced at a comparable graphization temperature. This finding has led to completion of the present invention.
In the Japanese Laying-Open Patent H-3-245458, there has been disclosed a high capacity carbonaceous material containing 0.1 to 2.0 wt % of boron. However, this publication fails to disclose the effect of addition of boron on d002 or on heat treatment temperature. The present invention is reached only by simultaneously employing two elements, that is control of the interstitial planar section density in the graphite having the looped closure structure as found previously by the present inventors, and addition of boron. A principal object of addition of boron in the present invention is to lower the temperature in the graphizing heat treatment, with the object of boron addition being slightly different from the object in the above-mentioned Publication. It is noted that the graphite material with a smaller d002 value can be obtained by heat treatment at a temperature lower than that used conventionally.